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Re: [libvirt] [Qemu-devel] Problem setting CPU topology



On Tue, Jul 10, 2012 at 2:02 PM, Eduardo Habkost <ehabkost redhat com> wrote:
> On Tue, Jul 10, 2012 at 11:54:05AM +0200, Christophe Fergeau wrote:
>> On Sat, Jul 07, 2012 at 07:10:53PM +0300, Zeeshan Ali (Khattak) wrote:
>> > Hi,
>> >    I'm trying to set exact CPU topology to qemu-kvm domains to match
>> > host's topology. In my case, host topology is: 1 socket, 2 cores and 2
>> > threads. If I set the XML like this:
>> >
>> > <domain type='kvm'>
>> >   ..
>> >   <vcpu placement='static'>4</vcpu>
>> >   <os>
>> >     <type arch='i686' machine='pc-0.15'>hvm</type>
>> >     <boot dev='hd'/>
>> >   </os>
>> >   <cpu mode='host-model'>
>> >     <model fallback='allow'/>
>> >     <topology sockets='1' cores='2' threads='2'/>
>> >   </cpu>
>> > ..
>> >
>> > The qemu commandline launched for this domain looks like this:
>> >
>> > /usr/bin/qemu-kvm -name fedora17-2 -S -M pc-0.15 -cpu
>> > core2duo,+lahf_lm,+rdtscp,+aes,+popcnt,+sse4.2,+sse4.1,+pdcm,+xtpr,+cx16,+tm2,+est,+smx,+vmx,+ds_cpl,+dtes64,+pclmuldq,+pbe,+tm,+ht,+ss,+acpi,+ds
>> > -enable-kvm -m 1152 -smp 4,sockets=1,cores=2,threads=2 -uuid
>> > c573342b-2876-05b8-098e-6d5314cab062 -nodefconfig -nodefaults -chardev
>> > socket,id=charmonitor,path=/home/zeenix/.config/libvirt/qemu/lib/fedora17-2.monitor,server,nowait
>> > -mon chardev=charmonitor,id=monitor,mode=control -rtc
>> > base=utc,driftfix=slew -no-kvm-pit-reinjection -no-shutdown -no-acpi
>>
>> I debugged this together with Zeeshan, and the issue comes from this -no-acpi flag
>> which libvirt added because the domain XML was missing
>> <features><acpi/></features>
>> So in the end that was a bug in Boxes, not really a libvirt/qemu issue.
>> Unless libvirt/qemu should be taught that CPU topology won't work without ACPI
>> and complain about it?
>
> I don't think it's really impossible for any guest OS to recognize the
> CPU topology or boot the other CPUs without ACPI. It looks like it's
> just a limitation of (most?) guest OSes. If that's the case, libvirt or
> QEMU can't prevent the user from running a (possibly valid)
> configuration just because it's not very common.
>
> --
> Eduardo
>

Isn't MPS and APIC support required for a guest OS to really handle
the topology correctly? Now days ACPI provides most of that
information so its likely that QEMU doesn't support plain old MPS +
APIC. Its also possible that QEMU does support this but the kernel got
confused because I see the CPUID +acpi specified in the command line
while -no-acpi is on the command line as you noted.

-- 
Doug Goldstein


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